1. Technical Field
The present invention relates to methods for forming a steel sheet by hot pressing to provide a final formed product hardened by plastically working, machining, and cooling a heated steel sheet in sequence.
2. Description of Related Art
In recent years, in the motor vehicle industry, there is a need for lightweight vehicle body structures providing a high level of safety, and in order to satisfy the need, attention has been given to a method for forming a steel sheet by hot pressing to obtain a strong vehicle body structure without increasing the sheet thickness. For example, in a method for forming a steel sheet by hot pressing according to Japanese Patent Publication No. 2005-248253 (paragraph 0022, FIG. 2), a final formed product is obtained from a steel sheet through a plurality of steps by using one die set. Specifically, a primary formed product is obtained by plastic working in which a steel sheet heated to high temperatures is pressed from above and below so as to be plastically deformed along pressing surfaces of a die set; a secondary formed product is then obtained by machining, e.g., by piercing the primary formed product; and then, the secondary formed product is cooled with the die set held at a bottom dead center position so as to be hardened, thereby obtaining a strong final formed product.
However, in the hot pressing of Japanese Patent Publication No. 2005-248253 (paragraph 0022, FIG. 2), plastic working, machining, and cooling are sequentially performed by using one die set, and thus, it takes considerable time to obtain one final formed product.
To address the problem, a method using a transfer press to increase productivity is contemplated. Specifically, a primary forming die set for primary forming in which plastic working is performed, a secondary forming die set for secondary forming in which machining is performed, a cooling die set for cooling in which cooling is performed are sequentially arranged in the transfer press so as to be synchronously operated. Steel sheets, primary formed products, and secondary formed products are sequentially fed to these die sets, thereby successively obtaining final formed products to increase productivity.
The transfer press is generally comprised of a mechanical press with high productivity. The mechanical press drives a flywheel with a motor, and the driving force of the flywheel is converted to linear motion by a crank mechanism. A clutch, a brake, etc., are provided on a power transmission path through which the driving force is transmitted, and the engagement of the clutch allows the driving force of the flywheel to be transmitted to the crank mechanism to move the die sets placed in the press up and down, and with application of the brake, the vertical movement of the die sets is stopped.
Incidentally, a cooling (hardening) step for a secondary formed product in hot pressing takes a longer time than the other two steps. Therefore, when a cooling die set is held at a bottom dead center position for a predetermined time period to cool a secondary formed product with a transfer press, the other die sets for primary forming and secondary forming are also held at corresponding bottom dead center positions for the same time period, and thus, primary and secondary formed products are held in contact with the corresponding die sets for a longer time period than necessary, thereby releasing the heat of each of the primary and secondary formed products to the corresponding die set. This release causes the temperature of the secondary formed product immediately before being carried into the cooling die set to be lower than the hardening start temperature at which hardening starts and which is required for hardening.
In order to avoid this, in a method for forming a steel sheet by hot pressing according to Japanese Patent Publication No. 2007-136533 (paragraph 0028, FIG. 1), primary and secondary formed products are heated or kept warm while being held in corresponding die sets to prevent the temperature of each of the primary and secondary formed products from being lower than the hardening start temperature.
However, in Japanese Patent Publication No. 2007-136533 (paragraph 0028, FIG. 1), a large amount of energy is consumed to manage the temperature of each of the primary and secondary formed products, and thus, the running cost, such as the electricity cost, increases.
With the use of a mechanical press for a cooling step in a transfer press, even when a brake is applied to stop a corresponding die set at a bottom dead center position, shock caused by the contact between upper and lower dies of the die set causes the stop location at which the die set stops to deviate slightly from the normal stop location corresponding to the bottom dead center position, and thus, when secondary formed products are sequentially held in the cooling die set under pressure, the conditions of contact between the secondary formed products and the cooling die set vary, and thus, the quality of final formed products may vary in continuous production.
Furthermore, while improvement in productivity is expected by simply increasing the speed of the vertical movement of the cooling die set, an increase in the operating speed of a press of the mechanical press causes the brake for stopping the vertical movement of the cooling die set to be less effective, and thus, the conditions of the contact between the secondary formed products held under pressure and the cooling die set may further vary.
It is therefore an object of the present invention to provide a method for forming a steel sheet by hot pressing which method reduces the energy consumption to a minimum to reduce the running cost, can successively provide final formed products of uniform quality, and furthermore offers high productivity.